Specific impulse

Specific impulse (often shortened to I_sp) is a way to describe how well a rocket performs. It is also used to describe how well a jet engine performs. It can be used to compare rockets of different sizes. It is a way to know how much force a given engine makes for each bit of fuel.^[1] To do this, one needs to know how much fuel is in the engine.^[2] A high specific impulse means that a rocket needs less fuel to perform as well. If the specific impulse is lower, it means that the rocket needs more fuel to perform as well.

Specific impulse is a useful value to compare rocket or jet engines. It is used much like miles per gallon or litres per 100 kilometres is used to compare cars. An engine with a higher specific impulse uses fuel better. This means that the same amount of fuel lets the rocket go faster after all fuel is used up.^[1]

A rocket engine with a higher specific impulse doesn't mean it's 'more powerful'. That is, it will not make the rocket accelerate harder. In fact, the designs for engines with the highest specific impulse, like ion engines, are the 'weakest' of all types of engines. They accelerate the rocket slowly but steadily over a long period of time using only a tiny amount of fuel. In a race between two rockets with the same amount of fuel and two different engines the one with the more powerful engine will take an early lead, but when it burns down all its fuel, the rocket with higher specific impulse will still have some fuel left and will continue to accelerate. Eventually it will overtake its counterpart and will win the race if the distance is big enough for it to use its long-term advantage.

1. ↑ ^{1.0 1.1} "What is specific impulse?". Qualitative Reasoning Group. Archived from the original on 4 July 2016. Retrieved 22 December 2009.
2. ↑ Benson, Tom (11 July 2008). "Specific impulse". NASA. Archived from the original on 24 January 2010. Retrieved 22 December 2009.